1. Field of the Invention
The present invention pertains to chlor-alkali cells used for the manufacture of chlorine and caustic. More particularly, the present invention pertains to means and methods for prolonging the useful life of diaphragms employed in chlor-alkali cells. Even more particularly, the present invention pertains to means and methods for unplugging the diaphragms used in chlor-alkali cells to thereby prolong the useful life of the diaphragm.
2. Prior Art
As is known to those skilled in the art to which the invention pertains, one of the more essential requirements during chlor-alkali cell operation is the maintenance of optimum diaphragm permeability. Maintenance of diaphragm permeability is most crucial where synthetic diaphragms are employed, especially those diaphragms formed from chemically stable perfluorinated compounds, such as those sold commercially under the names NAFION and GORE-TEX. When the permeability of the diaphragm falls below an accepted minimum, cell operation must be terminated and the diaphragm must be renewed or replaced.
In seeking maintenance of the permeability of the diaphragm, it is greatly more desirable to renew or rejuvenate a diaphragm than to replace the diaphragm. Thus, the prior art has recognized that a reduction in the permeability of the diaphragm is caused by the plugging thereof. The plugging is attributable to the accumulation of acid- and base- insoluble solids which arise as a result of degradation of the cell structural materials in the anolyte and by the precipitation of cationic impurities in the brine, e.g. hydroxides, carbonates and the like, which may be present in the anolyte. The former type of accumulation is referred to as "irreversible plugging." The latter type of accumulation is referred to as "reversible plugging" because the precipitates can be redissolved by chemical treatment.
It is to be appreciated that mere chemical treatment, alone, to dissolve the precipitates is insufficient since the dissolved cations in solution must be transported across the diaphragm against a flux of hydroxide ions. If the pH in the diaphragm is sufficiently high, certain of the dissolved cations, e.g. ferric and magnesium ions, could be reprecipitated. Thus, the prior art has sought and proposed ways of rejuvenating a diaphragm while concommitantly overcoming the problems alluded to herein. For example, in U.S. Pat. No. 3,630,863 there is taught the electrical disconnection and brine flow shut-off of a chlor-alkali cell followed by the introduction into the anolyte of gluconic acid or a similar water soluble hydroxy carboxylic compound.
In U.S. Pat. No. 3,467,586 there is taught the rejuvenation of a diaphragm by a process comprising brine flow shut-off, electrical disconnection of the cell, drainage of the cell and the acid treatment of the diaphragm. Useful acids include hydrochloric acid, acetic acid, formic acid, sulfuric acid and the like. U.S. Pat. No. 583,330 teaches the addition of hydrochloric acid directly to the anolyte during electrolysis. Other background art may be found in U.S. Pat. No. 3,485,730.
The present invention, on the other hand, permits the rejuvenation of a diaphragm while obviating the need for shutting down the cell.